Frequently, there is a need to measure the vacuum level of a package. For example, a MicroElectroMechanical System (MEM) is typically a very small device that can be used to sense pressure, temperature, chemical, vibration, light, among other things, in packaging devices (MEM's package). There is a need for maintaining a vacuum in MEMs packages. Ion gauges can be used, among other things, for measuring the vacuum level by measuring the ratio between an ion current and an electrical current, as will become more evident.
One type of ion gauge uses vacuum tubes. However, vacuum tube type ion gauges are too big and bulky to be used for measuring the level of vacuum in a MEMs package where the internal volume is small. Further, vacuum tube type ion gauges are expensive.
Alternatively, MEMs ion gauges, which typically are small and inexpensive to manufacture, can be used for measuring the level of vacuum in a MEMs packages. FIG. 1A depicts a cross-section view of a conventional electronic portion of a MEMs ion gauge and FIG. 1B depicts a side view of a conventional electronic portion of a MEMs ion gauge. The electronic portion of a MEMs ion gauge 100 typically includes hot filaments 122, bias grids 124, and ion collectors 126. As depicted in FIG. 1A, the ion collectors 126 are shielded from the hot filaments 124 and the bias grids 124 because they (122, 124, 126) are in a single plane. The filaments 122, bias grids 124 and ion collectors 126 extend over a trench 110 and are coupled to an ion gauge substrate 140. Bond pads 130 provide electric connection to the ion gauge 100. The ion collectors 126 can be negatively charged, for example, by using −40 Volts. The bias grids 124 can be positively charged, for example, by using +100 Volts.
FIG. 2A depicts a top-down view of a conventional electronic portion of a MEMs ion gauge 100. FIG. 2B depicts a side view of a conventional electronic portion of a MEMs ion gauge 100. Referring to FIGS. 2A, 2B, the hot filaments 122 boil off electrons (e.g., “e−”), the positively charged bias grids 124 accelerate the electrons, and the negatively charged ion collectors 126 gather the positively charged ions (e.g., “ions+”). Referring to FIG. 2B, the ions are generated when the highly accelerated electrons collide with the residual gas molecules so that the ratio of ion current to the electron current at the bias grids 124 (e.g., lion Iion/Ielectron) is inversely proportional to the vacuum level. Therefore, the ion current can be expressed as in equation 1 depicted below:Ii=IePaL  (1)
Thus, the ion current (Ii) is proportional to the electron current Ie, times the cavity pressure P, times the ionization rate a, times the electron path length L.
However as will be explained in more detail, the typical ion gauge 100 as depicted in FIGS. 1A, 1B, 2A, 2B does not accurately measure the vacuum level in a package. For these and other reasons, an ion gauge that accurately measures the vacuum level is desired. An ion gauge that is small would also be valuable. An ion gauge that is inexpensive to manufacture would also be valuable. An ion gauge that is small, inexpensive to manufacture, and that accurately measures the vacuum level would also be valuable.